Prestressed concrete structure



March 25, 1952 L. COEF 4 2,590,685

PRESTRESSED CONCRETE STRUCTURE Filed Dec. 4.- 1947 2 15 13 WW I I v A4 YWWWWW A? Has 13 '11 12 11 INVENTOR.

LEO COFF AGENT Writente r. 5, 5

PRESTRESSED CONCRETE STRUCTURE Leo CoiT,,New York, N. Y.

' Application December 4, 1947, Serial No. 789,704

In Great Britain February 6, 1947 contraction (whether due totemperature changes or toshrinkage) will generally lead to the forma='tion 01 undesirable cracks. This phenomenon is due to the so-cailedsubgrade drag, which opposes to the free contraction of the slab atensile force proportional to the weight 01 the contracting portion plusany load thereon and also to the coemcient of friction between the slaband the ground. Sinceconcrete can take but little tension even ifconventionally reinforced, cracks will result; although thereinforcements will tend to prevent these cracks from widening, they maystill become the seat of appreciable quantities of moisture which,particularly under freezing conditions, may lead to the earlydeterioration of the slab.

To counteract this tendency to crack. it has been the practice tosubdivide each slab into numerous portions each of relatively smallweight, whereby the aforementioned tensile stresses will be reduced. Thejoints existing between adjacent portions, however, have proved anewsource of trouble, requiring considerable maintenance and care. Thefilling of these joints'with mastic or similar substances has never beenfully satisfactory, resulting in surface irregularities at both high andlow temperatures. Also the transfer of forces at the joints, effected inmost cases by dowels, has proved unsatisfactory.

an object of the invention is to provide a method of making concreteslabs of relatively large area, without the need for subdividing theslab into portions spaced horizontally.

Another object oi the invention is to provide a slab in which theaforesaid joints are eliminated or at least substantially reduced innumber.

A further object of the invention is to provide a road slab for ahighway or airfield runway presenting a substantially unbroken surfaceover a wide area.

in accordance with the invention, the problem of preventing the subgradedrag from cracking concrete is solved not by reducing the weight theconcrete through the provision of vertical joints, but by reducing thecoefllcient of friction between the slab and its support, as a firststep, and by increasing the tensile strength of the'concrete, asa secondstep. To carry out the first measure, I provide a foundation or base ofsuit- 5 Claims. (o1. 94-4) able material which rests on the ground andmay expand and contract at will, any cracks developing therein beingunimportant since they will not be exposed. On top of this base I placethe slab, and between the base and the slab I insert an agent which willreduce the coeflicient of friction of the concrete with respect to thematerial of the base. Such an agent may be a layer of paraflin to whichmay be added a layer of tar paper or the like bonded to the slab butslidable on the paramned foundation; yet the coefficient of friction mayalso be reduced by the insertion of one or more metal sheets or thelike. In this manner it will be possible to reduce the force opposingcontraction to tolerable magnitudes, even where the weight of the slabis relatively great.

In order to increase the tensile strength of the slab, the latter may beprestressed by various methods in either one or two directions. This maybe accomplished by means of wire strands imbedded in the concrete, thesestrands being placed under initial stress as by mechanically tensioningthe sheathed strands (e. g. at the ends) and anchoring the ends of thewires to the slab, and/or by the use of expanding concrete. A1-ternatively, the wires can be tensioned against outside anchors beforepouring the slab, and released against the concrete after the latter hashardened. It should be noted that the base or foundation may likewise beeither reinforced or I prestressed; if desired, for the purpose ofminimizing or avoiding crack formations in the latter.

The above and other features and objects will become more fully apparentand the invention will be better understood from the followingdescription, taken in conjunction with the accompanying drawing inwhich:

Fig. 1 is a longitudinal sectional elevation of a road slab according tothe invention;

Fig. 2 is a transverse sectional elevation of the slab shown in Fig. i;

Fig. 3 is a view similar to Fig. 2, showing a modified slab;

Fig. 4.-

a fragmentary perspective view of the slab shown in Fig. 3, illustratinga method 0 making the same; and

Fig. 5 is a view similar to Fig. 1, showing a further modification.

As shown in Fig. i, there is supported on the ground it a base orfoundation it having a sliding surface it. The base it may be a screedof cinder concrete, a layer of evenly aligned terracotta tiles and soforth. The surface it is preferably paraffined and carries a layer iiiwhich may be tar paper bonded to the slab it resting thereon.

' form the slab ll.

wires and a set of transverse wires IS. The

wires IE, it run in sheaths or pipes I! so as not tobe bonded to theconcrete, and are maintained under tension by means such as nuts llengaging the threaded ends I 9 of the wires. serve to distribute thecompressive force exerted upon the slab by the tendency of the wires toregain their original length.

According to a further feature of the invention, buckling of the slab llunder pressure from the anchor plates 20 may be prevented by placing thetransverse wires i6 alternately above and below the longitudinal wiresl5 so as to form points of contact 2i therewith. Experiments have shownthat this expedient reduces the effective buckling length of the slab tothe distance between adjacent points of contact 2|, thus preventingbuckling even for relatively great longitudinal stresses.

According to still another feature of the invention, the subgrade dragmay be reduced by diminishing the weight of the slab without, however,resorting to vertical jointing. This may be accomplished, as shown inFig. 3, by providing the slab ll with a plurality of cavities 22 whichare formed between longitudinal ribs 23 andtransverse ribs 24. The ribs23, 24 may have imbedded therein the sheaths I! for the wires l5 and I6,respectively, as best seen in Fig. 4, although it is to be understoodthat these wires may also extend across the cavities 22.

Fig. 4 illustrates a method of manufacturing the slab i4 shown in Fig.3. It will be seen that the slab is originally formed with a bottomlayer 25 carrying the intersecting ribs 23, 24, said ribs being providedwith stirrups 26 to accommodate permanent forms of wire lath 21 ofconvex shape. The forms 21 serve as a support for the upper layer 28which is poured last.

The members 23, 24, 25 of the slab l4 may be either poured or precast,it being understood that in the latter case the slab may be composed ofsections (as indicated by the chain-dotted dividing line 29) which aresubsequently joined together by the common poured top layer 28, wherebya monolithic structure is formed.

Furthermore, a slab of the type disclosed in Fig. 3 may be composed ofseparate wide-flange sections, such as I-sections or channels, stressedtogether by wires or strands so as to coact monolithically. Such anarrangement is shown in Fig. 5.

In this figure, 'a plurality of wide-flange I- sections 30 are placedalongside one another to Wires lia, l5b, running longitudinally of theslab as before but extending transversely of the sections 30, serve tohold the sections together while at the same time prestressing the slabin one direction, the prestressing in a direction perpendicular theretobeing accomplished by the wires l6 extending within the webs ofrespective sections. Generally, the

members 30 will be precast in which case no sheathing of the wires willbe necessary. It is to be understood that the abutments 3| betweenadjacent slabs are distinct from the vertical joints previcuslymentioned, since the sections 30 are not spaced from one another but arefirmly Anchor plates 4 stressed together whereby the layer It" will actlike the monolithic structure shown in the preceding figures.

Although the concrete structure in accordance with the invention hasbeen described in the foregoing as comprising a slabsupported on afoundation or base, it will be understood .that the invention may alsobe considered as providing a method of making an extended, substantiallyunbroken concrete surface by forming a slab, dividing said slab along ahorizontal plane (l2) to form two superposed portions (Ii and l3, l4),and reducing the coefiicient of friction between saidtwo portions. Theupper portion may be made hollow to reduce the weight thereof, as by theprovision of cavities 22, and may be prestressed in any suitable manner.

Furthermore it is emphasized that the embodi ments shown and describedserve merely for purposes of illustration rather than limitation,numerous modifications and adaptations thereof being possible withoutexceeding the spirit and scope of the invention as defined in theobjects and in the appended claims. 4

I claim:

l. A concrete structure comprising a solid foundation having an evenhorizontal surface, a substantially unbroken concrete slab resting onsaid surface, said foundation being at least coextensive with said slab,anti-friction means substantially coextensive with said slab andinterposed between said foundation and said slab, and prestressing meansin said slab maintaining same under horizontal compression.

2. A concrete structure comprising a solid foundation having an evenhorizontal surface. a substantially unbroken concrete slab resting onsaid surface, said foundation being at least coextensive with said slab,anti-friction means substantially coextensive with said slab andinterposed between said foundation and said slab, and first and secondprestressing means in said slab maintaining same under horizontalcompression, said first and second prestressing means extending atsubstantially right angles to each other.

3. A concrete structure comprising a solid foundation having an evenhorizontal surface, a substantially unbroken concrete slab resting onsaid surface, said foundation being at least coextensive with said slab,said slab being provided with internal cavities, anti-friction meanssubstantially coextensive with said slab and interposed between saidfoundation and said slab, and prestressing means in said slabmaintaining same under horizontal compression.

4. A concrete structure comprising a solid foundation having an evenhorizontal surface, a plurality of elongated concrete members positionedon said surface alongside one another, prestressing means extendingtransversely of said elongated members and stressing same together,thereby eflectively forming an unbroken slab, said foundation being atleast coextensive with said slab, and anti-friction means substantiallycoextensive with said slab and interposed between said foundation andsaid elongated members.

5. A concrete structure comprising a solid foundation having an evenhorizontal surface, a plurality of elongated concrete profiles havingcoextensive upper and lower flanges, said profiles being positioned onsaid surface alongside one another with corresponding flanges abutting,prestressing means extending transversely of said elongated profiles andstressing same together, thereby eifectively forming an unbroken slab 5having internal cavities, said foundation being at least coextensivewith said slab, and anti-friction means substantially coextensive withsaid slab and interposed between said foundation and said profiles.

LEO COFF.

REFERENCES CITED The following references are of record in the e of thispatent:

UNITED STATES PATENTS Number Name Date 1,650,554 Wait Nov. 22, 19271,684,663 D111 Sept. 18, 1928 1,818,254 Hewett Aug. 11, 1931 2,155,121Finsterwolder Apr. 18, 1939 2,315,634 McCall Apr. 6, 1943 6 Number NameDate 2,323,848 Schafier July 6, 1943 2,329,189 D111 Sept. 14, 19432,329,670 Valles Sanchez Sept. 14, 1943 2,425,079 Billig Aug. 5, 1947FOREIGN PATENTS Number Country Date 557,025 Great Britain Nov. 2, 1943OTHER REFERENCES Joints for Concrete Pavements," by Mensch,

